Solubility data for the Na2CO3 + Na2SO4 + H2O2 + H2O system at 293 K were determined by Schreinmaker's method. The results were used to construct a phase diagram in the oblique projection according to the Janeck method. At constant temperature, when the hydrogen peroxide concentration is below 50 % (mass fraction), this diagram includes three invariant points, six double saturated liquid curves, and four crystallization fields corresponding to Na2CO3 (.) 10H(2)O, Na2SO4 (.) 10H(2)O, Na2CO3 (.) 1.5H(2)O(2), H2O, and Na2SO4, 0.5H(2)O(2) (.) H2O.